Wireless cellular communication networks incorporate a number of mobile UEs and a number of NodeBs. A NodeB is generally a fixed station, and may also be called a base transceiver system (BTS), an access point (AP), a base station (BS), or some other equivalent terminology. As improvements of networks are made, the NodeB functionality evolves, so a NodeB is sometimes also referred to as an evolved NodeB (eNB). In general, NodeB hardware, when deployed, is fixed and stationary, while the UE hardware may be portable.
User equipment (UE), also commonly referred to as a terminal or a mobile station, may be a fixed or mobile device and may be a wireless device, a cellular phone, a personal digital assistant (PDA), a wireless modem card, and so on. Uplink communication (UL) refers to a communication from the UE to the NodeB, whereas downlink (DL) refers to communication from the NodeB to the UE. Each NodeB contains radio frequency transmitter(s) and the receiver(s) used to communicate directly with the UE, which may move freely around it. Similarly, each UE contains radio frequency transmitter(s) and the receiver(s) used to communicate directly with the NodeB. In cellular networks, the UE cannot communicate directly with each other but have to communicate with the NodeB.
Long Term Evolution (LTE) wireless networks, also known as Evolved Universal Terrestrial Radio Access (E-UTRA), are being standardized by the 3GPP working groups (WG). OFDMA (orthogonal frequency division multiple access) and SC-FDMA (single carrier FDMA) access schemes were chosen for the down-link (DL) and up-link (UL) of E-UTRA, respectively. User equipments are time and frequency multiplexed on a physical uplink shared channel (PUSCH), and a fine time and frequency synchronization between UE's guarantees optimal intra-cell orthogonality. In case the UE is not UL synchronized, it uses a non-synchronized Physical Random Access Channel (PRACH), and the Base Station provides back some allocated UL resource and timing advance information to allow the UE to transmit on the PUSCH. The general operations of the physical channels are described in the EUTRA specifications, for example: “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (TS 36.211 Release 8, or later).”
Several types of physical channels are defined for the LTE downlink. One common characteristic of physical channels is that they all convey information from higher layers in the LTE stack. This is in contrast to physical signals, which convey information that is used exclusively within the physical (PHY) layer. Currently, the LTE DL physical channels are as follows: Physical Downlink Shared Channel (PDSCH), Physical Broadcast Channel (PBCH), Physical Multicast Channel (PMCH), Physical Control Format Indicator Channel (PCFICH), Physical Downlink Control Channel (PDCCH), and Physical Hybrid ARQ Indicator Channel (PHICH).
A reference signal (RS) is a pre-defined signal, pre-known to both transmitter and receiver. The RS can generally be thought of as deterministic from the perspective of both transmitter and receiver. The RS is typically transmitted in order for the receiver to estimate the signal propagation medium. This process is also known as “channel estimation.” Thus, an RS can be transmitted to facilitate channel estimation. Upon deriving channel estimates, these estimates are used for demodulation of transmitted information. In downlink transmission, two types of reference signals are available. The first type of reference signal is un-precoded and is transmitted over the entire system bandwidth of a cell, and is generally referred to as cell-specific reference signal (CRS). Another type of reference signal is modulated by the same precoder as applied on the data channel, and therefore enables a UE to estimate the effective precoded MIMO channel characteristics. This type of RS is sometimes referred to as De-Modulation RS or DMRS. DMRS is transmitted only when a UE is being scheduled, and is therefore only transmitted over the frequency resource assignment of data transmission. DMRS can also be applied in uplink transmission (PUSCH), in case a UE transmitter is equipped with multiple antennas. An RS may also be transmitted for other purposes, such as channel sounding (SRS), synchronization, or any other purpose. Also note that the Reference Signal (RS) can be sometimes called the pilot signal, or the training signal, or any other equivalent term.
The LTE PHY can optionally exploit multiple transceivers and antenna at both the base station and UE in order to enhance link robustness and increase data rates for the LTE downlink. Spatial diversity can be used to provide diversity against fading. In particular, maximal ratio combining (MRC) is used to enhance link reliability in challenging propagating conditions when signal strength is low and multipath conditions are challenging. Transmit diversity can be used to improve signal quality by transmitting the same data from multiple antennas to the receiver. Spatial multiplexing can be used to increase system capacity by carrying multiple data streams simultaneously from multiple antennas on the same frequency. Spatial multiplexing may be performed with one of the following, for example: cyclic delay diversity (CDD) precoding methods: zero-delay, small-delay, or large-delay CDD. Spatial multiplexing may also be referred to as MIMO (multiple input multiple output).
With MRC, a signal is received via two (or more) separate antenna/transceiver pairs. The antennas are physically separated, and therefore have distinct channel impulse responses. Channel compensation is applied to each received signal within the baseband processor before being linearly combined to create a single composite received signal. When combined in this manner, the received signals add coherently within the baseband processor. However, the thermal noise from each transceiver is uncorrelated, resulting in improved signal to noise ratio (SNR). MRC enhances link reliability, but it does not increase the nominal maximum system data rate since data is transmitted by a single antenna and is processed at the receiver via two or more receivers.
MIMO, on the other hand, does increase system data rates. This is achieved by using multiple antennas on both the transmitting and receiving ends. In order to successfully receive a MIMO transmission, the receiver must determine the channel impulse response from each transmitting antenna. In LTE, channel impulse responses are determined by sequentially transmitting known reference signals from each transmitting antenna. While one transmitter antenna is sending the reference signal, the other antenna is idle on the same time/frequency resources. Once the channel impulse responses are known, data can be transmitted from both antennas simultaneously. The linear combination of the two data streams at the two receiver antennas results in a set of two equations and two unknowns, which are resolvable into the two original data streams.
Physical channels are mapped to specific transport channels. Transport channels are service access points (SAPs) for higher layers. Each physical channel has defined algorithms for bit scrambling, modulation, layer mapping, precoding, and resource assignment. Layer mapping and precoding are related to MIMO applications. Basically, a layer corresponds to a spatial multiplexing channel. Channel rank can vary from one up to the minimum of number of transmit and receive antennas. For example, given a 4×2 system, i.e., a system having four transmit antennas and two receive antennas, the maximum channel rank is two. The channel rank associated with a particular connection varies in time and frequency as the fast fading alters the channel coefficients. Moreover, the channel rank determines how many layers, also referred to as the transmission rank, can be successfully transmitted simultaneously. For example, if the channel rank is one at the instant of the transmission of two layers, there is a strong likelihood that the two signals corresponding to the two layers will interfere so much that both of the layers are erroneously detected at the receiver. In conjunction with precoding, adapting the transmission to the channel rank involves striving to use as many layers as the channel rank. Layer mapping specifies exactly how the extra transmitter antennas are employed. For non-codebook based precoding, the precoding applied for the demodulation reference signal (DMRS) is the same as the one applied for the PDSCH (for uplink) and PDSCH (for downlink). Multiplexing of the demodulation reference signals can be time-division multiplexing, frequency division multiplexing, code division multiplexing or a combination of them.
Precoding is used in conjunction with spatial multiplexing. The basic principle involved in precoding is to mix and distribute the modulation symbols over the antennas while potentially also taking the current channel conditions into account. Precoding can be implemented by, for example, multiplying the information carrying symbol vector containing modulation symbols by a matrix which is selected to match the channel based on a certain selection criterion. Some examples of selection criterion include average throughput and maximum signal-to-interference-noise ratio (SINR). Sequences of symbol vectors thus form a set of parallel symbol streams and each such symbol stream is referred to as a “layer”. Thus, depending on the choice of precoder in a particular implementation, a layer may directly correspond to a certain physical antenna or a layer may, via the precoder mapping, be distributed onto several physical antennas.
In LTE Rel-8, single layer beamforming on antenna port 5 is already supported. Single-layer beamforming is based on non-codebook precoding and relies on a dedicated demodulation reference symbol (DMRS) for data demodulation. DMRS symbols are precoded with the same precoding matrices as the PDSCH data symbols and therefore enable UE to estimate the “effective” channel after precoding. Rank-1 transmission is enforced. A UE is restricted to receive a single transport block (codeword) which is mapped to one layer (data stream) in DL transmission. From the UE's perspective, the effective 1-layer channel appears as if data is transmitted from a single virtual antenna. DMRS corresponding to this layer is defined as antenna port 5 in LTE Rel-8 to enable channel estimation.
A very simple multi-user MIMO (MU-MIMO) scheme is currently supported in 3GPP LTE Rel-8 specification. A higher-layer configured semi-static MU-MIMO mode is configured so that a UE knows that the eNB will attempt to schedule it with one or multiple other UEs. Codebook-based precoding is used where the precoding matrices for a UE are selected from a pre-defined set (i.e. codebook) of fixed matrices (i.e. precoding matrices/vectors). CQI feedback is important for informing the DL channel status to the eNB in order to perform accurate DL link adaptation (e.g. rank, precoding matrices, modulation and coding schemes, frequency-selective scheduling) and UE scheduling. Since a UE does not know which other UEs it will be scheduled together and what precoding matrices will be used for the co-schedule UE, CQI feedback in Rel-8 is based on single-user MIMO (SU-MIMO) precoding. Due to such a constraint, CQI report for MU-MIMO mode in Rel-8 is highly unreliable, which consequently limits the performance of Rel-8 MU-MIMO.